Metallocene catalysts allow the production of polyolefins with unique properties such as narrow molecular weight distributions. These properties in turn result in improved structural performance in products made with the polymers, such as greater impact strength and clarity in films. While metallocene catalysts have yielded polymers with improved characteristics, they have presented new challenges when used in traditional polymerization systems.
For example, when metallocene catalysts are used in fluidized bed reactors, “sheeting” and the related phenomena “drooling” may occur. See U.S. Pat. Nos. 5,436,304 and 5,405,922. “Sheeting” is the adherence of fused catalyst and resin particles to the walls of the reactor. “Drooling” or dome sheeting occurs when sheets of molten polymer form on the reactor walls, usually in the expanded section or “dome” of the reactor, and flow along the walls of the reactor and accumulate at the base of the reactor. Sheeting and drooling may be a problem in commercial gas phase polyolefin production reactors if the risk is not properly mitigated. The problem is characterized by the formation of large, solid masses of polymer on the walls of the reactor. These solid masses or polymer (the sheets) may eventually become dislodged from the walls and fall into the reaction section, where they may interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning.
Various methods for controlling sheeting have been developed. These often involve monitoring the static charges near the reactor wall in regions where sheeting is known to develop and introducing a static control agent into the reactor when the static levels fall outside a predetermined range. See for example, U.S. Pat. Nos. 4,803,251 and 5,391,657. The static charge may be monitored with a static probe or voltage indicator. See for example, U.S. Pat. Nos. 4,532,311; 4,855,370; 5,391,657; and 6,548,610. Conventional static probes are described in U.S. Pat. Nos. 4,532,311; 5,648,581; and 6,008,662.
Other background references include U.S. Patent Application Publication No. 2002/103072U.S; U.S. Pat. Nos. 5,066,736; 5,126,414; 5,283,278; 5,332,706; 5,427,991; 5,461,123; 5,473,028; 5,492,975; 5,610,244; 5,627,243; 5,643,847; and 5,661,095; PCT Publications WO 96/08520; WO 97/06186; WO 97/14721; WO 97/15602; WO 97/27224; WO 99/61485; WO 2005/068507; and European Publications EP-A 1 0 549 252; EP 0 811 638 A; and EP 1 106 629 A.
Various antistatic agents, static control agents, and process “continuity additives” are disclosed in U.S. Patent Appl. Pub. No. 2005/0148742, U.S. Pat. Nos. 4,012,574; 4,555,370; 5,034,480; and 5,034,481; European Publications EP 0229368 and EP 0 453116, and PCT Publications WO 96/11961 and WO 97/46599. U.S. Patent Appl. Pub. No. 2008/027185, discloses the use of aluminum stearate, aluminum distearate, ethoxylated amines, mixtures of polysulfone copolymer, polymeric polyamine, and oil-soluble sulfonic acid, as well as mixtures of carboxylated metal salts with amine-containing compounds that may also be used to control static levels in a reactor.
Static control agents, including several of those described above, may result in reduced catalyst productivity. The reduced productivity may be as a result of residual moisture in the additive. Additionally, reduced productivity may result from interaction of the polymerization catalyst with the static control agent, such as reaction or complexation with hydroxyl groups in the static control agent compounds. Depending upon the static control agent used and the required amount of the static control agent to limit sheeting, loss in catalyst activities of 40% or more have been observed.
Therefore, there exists a need for useful additives for controling static levels, and thus sheeting, in fluidized bed reactors, especially for use with metallocene catalyst systems.